Fabrication of optical waveguides using slurry deposition

ABSTRACT

A tubular formation of particulate optical material is formed by a layer slurry deposition process which involves spraying layer after layer of slurry containing particles of the optical material onto a rotating rod-shaped bait. The composition of the slurry and particularly the index of refraction of the optical material may be varied from one layer to another or from one group of layers to another to obtain a graded or stepped refraction index profile in the formation, in an optical preform formed therefrom, and ultimately in the fiber drawn from the optical preform. The particles of the slurry are suspended in a liquid vehicle which evaporates in the spray stream or shortly after deposition, and are coated with an organic binder which holds them together in the layer and in the formation, so that the formation is self-supporting. Then, the organic binder is removed and the formation is sintered, followed by a collapse of the sintered tubular formation into the optical preform in the form of a solid rod. Then, optical fiber of the desired optical properties can be drawn from the optical preform. The associated apparatus includes a spray gun aimed at the bait rod, and a receptacle for the slurry from which the slurry is supplied to the spray gun. The bait rod is clamped in chucks and is rotated, while the spray gun moves in the axial direction of the bait rod in a plurality of passes to deposit the formation on the bait rod.

BACKGROUND OF THE INVENTION

This invention relates to an improved process for forming opticalpreforms for the production of optical fibers. The process is especiallysuitable for making optical preforms which possess step or gradedrefractive index profiles. Such characteristics enable the production ofoptical fibers cables exhibiting reliable operating characteristics.

There has been a continuous search in the prior art for the economicaland mass production of fiber optic cables for use in opticalcommunications systems.

Thus, the prior art considered and describes techniques such as "soot"deposition or hydrolysis wherein a gas vapor mixture is hydrolyzed by aflame to form a glass precursor particulate. The particulate is thendeposited on a rotating glass rod serving as a mandrel. The soot isdeposited upon the mandrel in a perpendicular direction to providesuccessive layers of constant radius or to provide a composite articlewith radial gradations in the index of refraction by varying the dopantconcentration during successive passes of the burner flame. The mandrelis then removed and the thus released cylindrical article is collapsedto a solid rod-shaped preform and then a fiber is drawn from thispreform. This process is shown and discussed in U.S. Pat. No. 3,826,560and U.S. Pat. No. 3,823,995. This process, however, is very laboriousand time-consuming. Consequently, the preform and the fiber drawntherefrom are relatively expensive. Moreover, it is very difficult ifnot impossible to achieve a complete utilization of the soot or of thematerial from which it is obtained because of difficulties encounteredin capturing and/or recycling such materials.

Other techniques, such as that described in U.S. Pat. No. 3,614,197involve processes for continuously forming an optical fiber by using amulti-stepped funnel-shaped vessel to form a solid glass rod-shapedpreform which is then heated and drawn into a fiber. Even this procedureis rather expensive and prone to result in contamination of the preformand the fiber by undesirable inclusions.

In any event, there is a desire to provide a solid optical preform andthen draw or process the same into an optical fiber. Both the continuousforming process and the tubular preform forming and collapsing approachhave inherent benefits in the mass production of such cables but alsocertain disadvantages.

Furthermore, U.S. Pat. No. 3,966,446 discusses another technique forproviding an optical preform. The optical preform is here fabricated bythe axial deposition from a direction along the preform axis as opposedto radial deposition from a direction perpendicular to the preform axisas used in the above-mentioned approaches. This technique does notrequire a mandrel and thus avoids the need for collapsing a cylindricalpreform prior to drawing. Yet, even this technique is rather cumbersomeand time-consuming and, consequently, expensive. In most instances, theavoidance of the need for collapsing the preform is more than outweighedby the inconvenience of using such a complicated process and the expenseassociated therewith.

The preforms thus provided in the just mentioned patent may be providedwith longitudinal gradations in the index of refraction and thus serveto enhance certain types of mode conversions. However, this technique isnot readily suited for providing radial gradations in the index ofrefraction. This is an additional reason for not using this approach inthe fabrication of fiber with radially graded index of refraction.

In any event, there is a need to provide large optical preforms whichthen can be drawn into elongated optical fibers. There is a further needto provide an optical preform which can exhibit step, single mode orgraded index profiles in the radial direction to enable the resultantcable to be used to more efficiently transmit optical information in theform of digital or other signals.

It is known that optical fiber cables which possess a single mode ofoperation alleviate mode dispersion problems. It has been a problem toproduce reliable cables employing single mode operation in that theprior art techniques could not adequately control the composition of thecable. Moreover, it is difficult to assure that the operation will beconducted in the single mode under all operating conditions. Thus, manycables employ a multi-mode operation in using radial gradations in theindex of refraction. In these cables the difference in velocity frommode to mode compensates for the different path lengths and results in arelatively equal traversal time for all modes.

It is clear that, in order to efficiently employ a single mode or amulti-mode operation, one must carefully and accurately control thefabrication of the fiber to assure that the same is consistent informulation and hence, possesses repeatable and reliable operatingcharacteristics.

The current fabrication techniques of all optical fiber preforms arebroadly based on one fundamental principle, i.e. vapor phase deposition.For example, the reported processes are chemical vapor deposition (CVD),modified chemical vapor deposition (MCVD), outside vapor phase oxidation(OVPO), inside vapor phase oxidation (IVPO), and plasma-activatedchemical vapor deposition (PCVD). In all these processes, metal halides,such as pure or doped silicon halides, are converted at hightemperatures to the respective oxide particles and the chemicalconversion and deposition processes occur simultaneously. As mentionedbefore, such conventional processes are rather time-consuming andexpensive, especially because of the slow rate of growth of thedeposited layer and the need for performing such processes in carefullycontrolled atmospheres and at relatively high temperatures.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to avoidthe disadvantages of the prior art.

More particularly, it is an object of the present invention to develop amethod of making an optical preform, especially one having a radiallygraded index of refraction, which does not possess the disadvantages ofthe conventional methods of this kind.

Still another object of the present invention is to provide a method ofthe kind here under consideration which is inexpensive to perform,results in a relatively rapid formation of the optical preform, and canbe conducted at least predominantly under ambient conditions.

A concomitant object of the present invention is to devise an apparatuswhich is especially suited for performing the method of the presentinvention.

It is yet another object of the invention so to construct the apparatusas to be simple in construction, inexpensive to manufacture, easy touse, and reliable in operation nevertheless.

In pursuance of these objects and others which will become apparenthereafter, one feature of the present invention resides in a process formanufacturing an optical preform, this process comprising the steps ofproviding a bait; forming at least one slurry containing particles of anoptical material with a predetermined index of refraction suspended in aliquid vehicle; depositing at least one cohesive layer of the particleson the bait, including covering at least a portion of the bait with theslurry and evaporating the liquid vehicle; removing the cohesive layerfrom the bait; and sintering the cohesive layer into the preform.Advantageously, the depositing step includes directing at least onestream of the slurry against the bait, and conducting relative movementbetween the stream and the bait.

When the bait is elongated and has a circumferential surface centered onan axis, it is advantageous when the conducting step includes performingrelative movement between the bait and the stream of the slurry both inthe axial and in the circumferential direction of the bait. Aparticularly simple solution is obtained when the performing stepincludes rotating the bait about its axis and moving the stream axiallyof the bait.

The process of the present invention further advantageously includes thestep of repeating the depositing step to deposit an additional cohesivelayer of the particles on top of the cohesive layer deposited during theprevious depositing step. This expedient has particularly advantageousresults when the forming step includes forming a plurality of slurriescontaining particles of optical materials with different indexes ofrefraction including the predetermined index of refraction. Then therepeating step includes using a slurry selected from the plurality ofslurries which contains particles of a different index of refractionthan those deposited during the previous depositing step.

According to a further advantageous facet of the invention, the formingstep includes coating the particles with a binder which holds theparticles together subsequent to the evaporation of the liquid vehicle.Then, the inventive process further includes the step of removing thebinder from the cohesive layer at the latest immediately prior to thesintering step, especially by heating at least the cohesive layer to atemperature sufficient to expel the binder from the cohesive layer.

It is further advantageous when the process further comprises the stepof heating at least the deposited layer in a hydrogen-free atmosphereprior to the sintering step to reduce the hydroxyl contents of thelayer.

An apparatus for manufacturing an optical preform in accordance with theabove method preferably comprises means for mounting a bait; at leastone source of slurry containing particles of an optical material with apredetermined index of refraction suspended in a liquid vehicle; meansfor depositing at least one cohesive layer of the particles on the bait,including means for covering at least a portion of the bait with theslurry and for evaporating the liquid vehicle; and means for sinteringthe cohesive layer after its removal from the bait into the preform.Advantageously, the depositing means includes means for directing atleast one stream of the slurry against the bait, and means forconducting relative movement between the directing means and the bait.

The apparatus may advantageously be used with an elongated bait having acircumferential surface centered on an axis. Then, the conducting meansincludes means for performing relative movement between the bait and thedirecting means both in the axial and in the circumferential directionof the bait. A particularly simple construction results when themounting means mounts the bait for rotation about its axis and when theperforming means includes means for rotating the bait, and means formoving the directing means axially of the bait.

Thus, a new and low cost process of fabrication of otpical waveguides isprovided in which the chemical conversion to the oxides and thedeposition are carried out in two distinctly separate steps. Thisprocess involves a layer slurry deposition (LSD) in which a slurry ofsilica or similar optical material is deposited in consecutive layers ona rotating bait surface at room temperature. The thus coated bait isthen sintered to a preform from which optical fibers can subsequently bedrawn.

To obtain the slurry or each of the slurries, chemically pure silicaand/or doped silica powders are mixed with an organic vehicle(consisting of a solvent, binder and deflocculant). Viscosity of theslurry can be adjusted by controlling the ratio of powders to the liquidvehicle. By controlling the flow rate and adjusting the location of anatomizer which forms the directing means, the slurry or slurries will besprayed directly onto a rotating bait surface. When the slurry dropletscome out of the atomizer or spray gun, the solvent or liquid vehiclequickly evaporates in air and the solid silica particles coated with athin film of organics uniformly deposit onto the bait surface. Layerafter layer of different silica composition can be deposited to form agraded structure which will then be sintered followed by conventionalfiber drawing.

A flow diagram of the optical fiber manufacture including the LSDprocess is shown below: ##STR1##

The sintering and fiber drawing steps can be performed as parts of asingle process step.

BRIEF DESCRIPTION OF THE DRAWINGS

Above-mentioned and other features and objects of this invention willbecome more apparent by reference to the following description taken inconjunction with the accompanying drawing, in which:

FIG. 1 is an axial sectional view of a spray gun head suitable for usein the practice of the invention;

FIG. 2 is a diagrammatic side elevational view of an arrangementsuitable for depositing the slurry on a bait or on a previouslydeposited layer; and

FIG. 3 is a flow diagram of the steps of a preferred manner ofpractising the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Prior to or during the performance of the process of the presentinvention, powders of pure and/or doped silica or similar opticalmaterials will be dispersed preferably in a non-aqueous liquid medium orvehicle, composed of a solvent, binder and deflocculant to make aslurry. The viscosity of this slurry can be controlled by adjusting theamount of its various constituents. When this slurry is sprayed on asurface, the solvent which is highly volatile will evaporate fastleaving a flexible but dimensionally stable or cohesive film. Theparticle size of the solid powders may vary between submicron to severalhundred microns.

The constituents of the liquid vehicle, in general, should have thefollowing properties:

Solvent

Low boiling point and viscosity

Soluble with binders and deflocculants

Chemically inert to silica based powders

Binder

Should provide high strength to hold the silica particles together afterthe solvent evaporates

Should readily evaporate or burn at temperatures below the sinteringtemperature to leave the cohesive layer prior to the sintering

Deflocculant

Should uniformly disperse silica powders in the slurry

Should keep the powders in suspended state in the slurry.

Layers of films can be deposited one above the other consecutively witheach layer having different dopant concentration to grade the refractiveindex profile. The composite article made in this way will be finallysintered to optically transparent glass in a gradient furnace and drawninto fiber.

The slurry for use in the present invention may be prepared as follows.Uniformly doped silica powders will be prepared in large quantities byplasma spray, sol-gel, spray drying or other similar technique. Thesetechniques are so well known that they need not be discussed here. Theplasma spray technique will be superior to the other techniques, in thesense that it does not require any OH removal operation. Furthermore,relatively cheaper raw materials can be used due to preferentialvolatilization of the impurities in plasma flame. However, the othertechniques can also be used for producing the powders. In such a case,however, the cohesive layer or article is preferably heated in ahydrogen-free atmosphere to reduce the amount of hydroxyl groupstherein.

The liquid vehicle will be prepared from an acetone based solvent,polyvinyl butyral, polymethyl methacrylate or other type of binders, andglycerol tri-oleate, benzene sulfonic acids or other type ofdeflocculants. Silica based powders will be suspended at roomtemperature in the liquid vehicle and agitated to produce a uniformlydispersed slurry. Depending on the desired Ge or other dopantconcentration profile, the proper amount of silica/organic and dopedsilica/organic suspensions from two separate containers will betransferred to a mixing chamber.

The slurry will then be sprayed through a spray gun on a bait surface atroom temperature. A typical spray gun head suited for directing a streamor spray of the slurry onto the bait is shown in axial section in FIG.1, wherein the reference numeral 1 has been used to identify the spraygun head in its entirety. The spray gun head 1 includes an annularsupport member 2 which is provided with a plurality of flow passages 3for an entraining medium, preferably a gaseous medium such as air. Thesupport member 2 is provided in its center with an internally threadedbore 4 into which there is threaded an externally threaded portion 5 ofa slurry nozzle 6. The nozzle 6 defines an internal flow passage 7 forthe slurry which terminates in an outlet orifice 8. The support member 2also has an externally threaded portion 9 onto which there is threadedan internally threaded portion 10 of a confining sleeve 11.

A guiding part 12 is confined in the interior of the confining sleeve 11and bounds, together with the confining sleeve 11 and the nozzle 6, anair distributing compartment 13 or a plurality of such air distributingcompartments 13. The guiding part 12 may be of one piece with thesupport member 2. A main flow passage 14 for the entraining medium, or aplurality of such main flow passages 14, is defined between the internalsurface of the guiding part 12 and the external surface of the nozzle 6.This main flow passage communicates with the distributing compartment 13and converges in the direction of flow of the entraining medium until itterminates at the outlet orifice 8. In operation, the entraining mediumflows through the main flow passage 14 toward the region surrounding theoutlet orifice 8 of the slurry nozzle 6, where it entrains droplets ofthe slurry for joint travel therewith in the form of a spray 15.

The spray gun head 1 further includes a cap 16 which has a conical outersurface and which is retained in position relative to the confiningsleeve 11 by engagement with an inwardly projecting portion or bead 17of the confining sleeve 11 with the conical outer surface of the cap 16.The cap 16 may be of one piece with the guiding part 12, in which casethe engagement of the bead 17 with the cap 16 also retains the guidingpart 12 in position relative to the confining sleeve 11. In thisconstruction, the guiding part 12 need not be of one piece with thesupport member 2.

The outer surface of the guiding part 12 bounds an additional flowpassage 18, or a plurality of such additional flow passages 18, with theinternal surface of the confining sleeve 11. The flow passage 18 extendsfrom the distributing compartment 13 to the bead 17 where it issealingly closed by the latter. A flattening orifice 19, or a pluralityof such flattening orifices 19, extends from the additional flow passageor passages 18 to a front region of the guiding part 12 as considered inthe direction in which the spray gun head 1 is aimed. The orifice orplurality of orifices 19 is defined between the cap 16 and the guidingpart 12. The flattening orifice or the plurality of such orifices 19 isso oriented that the entraining medium issuing therefrom will reduce thespread of or flatten the spray 15. A clean dry source of compressed airwill preferably be used at a pressure between 60-100 lbs. to break upthe slurry as it leaves the orifice 8 of the fluid nozzle 6, atomizingit and keeping the slurry within a cone-shaped area. When the slurrydroplets come out of the spray gun , the solvent evaporates in air andthe solid silica particles coated with thin film of organics depositonto the bait surface. There the organics coating will serve as a binderto hold the particles in position relative to one another and thus toform a cohesive layer from such particles.

The bait surface and the spray gun arrangement can be made in severalways. One simple arrangement is shown in FIG. 2 where a thin bait rod 20is mounted in a lathe chuck assembly 21, 22. Since the process isconducted at room temperature, the bait rod 20 may be selected from awide variety of materials.

During the deposition process, a spray gun 23, which is shown toincorporate the spray gun head 1, will move at a controlled speed in atransverse manner as considered in the drawing, that is, axially of thebait rod 20. Layer after layer of the same or different silicacomposition may be deposited to form a uniform, stepped, or gradedrefractive index profile. The undeposited materials are collected and,to the extent feasible, recycled.

FIG. 2 also shows that a slurry 24 may be contained in a slurryreceptacle 25, where the optical material particles are mixed in anyconventional manner with the liquid vehicle and the deflocculant. In theillustrated construction, the slurry 24 forms an upper surface 26, anddry compressed air at appropriate pressure is admitted through an inletconduit 27 into the receptacle 25, where it will bubble through theslurry 24 and rise to the upper surface 26 thereof. This passage of airthrough the slurry 24 may be sufficient to disturb the slurry 24 to suchan extent that it may not be necessary to provide any additionalmechanical stirring or mixing means to maintain the particles insuspension.

The spray gun 23 is connected to the receptacle 25 by a conduit 28which, in the illustrated construction, is rigid. The conduit 28 extendsthrough the upper level 26 of the slurry 24 to the bottom region of thereceptacle 25. The pressure of the air present above the upper level 26of the slurry 24 is at least sufficient to overcome the resistanceencountered by the slurry 24 on its way to the spray gun head 1 due tothe friction within it and between the slurry 24 and the internalsurface of the conduit 28 and to the elevation difference between theupper level 26 and the spray gun head 1. Thus, the slurry 24 will riseat least to the orifice 8, (see FIG. 1) if not squirt beyond it, whereit will be entrained for joint travel by the entraining gaseous mediumto form the spray 15. This spray 15 eventually reaches the exposedsurface of the bait 20 and forms a layer 29 thereon. The solvent orliquid vehicle is so volatile that it will evaporate, in most instances,while the particles of the optical material are still in transit in thespray 15. What remains is a thin film of the binder on the particles,which attaches such particles to one another in the just formed layer29. This makes the layer 29 cohesive, so that the particles will stay inthe layer 29.

In the construction illustrated in FIG. 2, the conduit 28 is rigid andis immovably connected to the receptacle 25. The receptacle 25, however,is mounted for movement axially of the bait rod 20, as indicated by thedouble-headed arrow. As the receptacle 25 is moved, by conventionalmoving means which has been omitted from the drawing, in the axialdirection of the bait rod 20, the conduit 28 and the spray gun 23 withits spray gun head 1 mounted thereon also move in the axial direction ofthe bait rod 20 so that the spray 15 deposits the slurry 24 or thecoated particles of the slurry first at one end of the exposedcircumferential surface of the bait rod 20 and then progressively atadjacent regions of the exposed circumferential surface until the otherend of the exposed circumferential surface is reached. Of course, thespray 15 is aimed on such an exposed surface, preferably along a planeincluding the longitudinal axis of the bait rod 20. However, it is alsopossible and contemplated for the receptacle 25 to be stationary, forthe conduit 28 to be flexible, and for only the spray gun 23 with itshead 1 to be movable in the axial direction of the bait rod 20.

A single layer 29 is deposited during each pass of the head 1 or of thespray 15 along the bait rod 20. To obtain uniformity in the depositedlayer 29, it is advantageous if not mandatory to cover the exposedsurface of the bait rod 20 around its entire circumference during thepass. In the illustrated construction, this is achieved by rotating thebait rod 20 about its longitudinal axis by rotating the chucks 21 and 22which grip the bait rod 20 outside of the exposed surface, that is,outside of the area reached by the spray 15. Depending on the characterof the movement of the spray head 1, the layer 29 will be deposited insuccessive overlapping annuli or in a continuous helical overlappingstrip.

A single layer 29 is usually not sufficiently thick either to beself-supporting or to contain a sufficient amount of the particulateoptical material to make an optical preform therefrom. Moreover, forobvious reasons, the layer 29 will have to have the same index ofrefraction throughout. For these reasons, the depositing operation isrepeated in a plurality of passes, each of them resulting in thedeposition of one coherent layer 29, until a formation 30 is provided onthe bait rod 20. The formation 30 is self-supporting, due to theadhesion of the particles of the layers 29 to one another due to theaction of the binder coating, both within each layer 29 and as betweenthe layers 29. The formation 30 also includes the requisite amount ofmaterial to form an optical preform of the required size therefrom.

It will be appreciated that each of the passes of the spray 15 is anoperation independent of the preceding and succeeding passes. Hence, inaccordance with the present invention, it is contemplated to vary thecomposition of the slurry 24, if not from pass to pass, then from agroup of passes to the next succeeding group of passes. In this respect,the main if not only variation is in the composition of the particulatematerial of the slurry 24 such that the index of refraction of thelayers 29 of the formation 30 varies from one layer 29 to another orfrom one group of layers 29 to another. In the first instance, aradially graded refractive index profile is obtained in the formation30, while the formation 30 has a radially stepped refractive indexprofile in the second instance. Each of these approaches hasrepercussions on the refractive index profile of the fiber drawn fromthe final preform such as to make the drawn fiber suitable for theintended use thereof.

As mentioned before, the formation 30 is self-supporting. Thus, at theend of the depositing operation, the bait rod 20 can be removed from theinterior of the formation 30. Then, the formation 30 will be furtherhandled to convert the same into a rod-shaped optical preform from whichan optical fiber can be drawn. This further handling includes heatingthe formation 30 to a temperature of about between 500° and 600° C., atwhich the formation 30 still retains its basic particulate and porouscharacter but the binder will either evaporate or become coverted ingaseous chemical compounds, such as by burning, so that it will escapefrom the formation 30 without a trace through the pores present betweenthe particles. Yet, the formation 30 will remain self-supporting andretain its tubular shape, apparently due to point fusion of theparticles and/or frictional and other mechanical forces between theparticles. Thereafter, the formation 30 may be passed, if needed,through a zone containing hydrogen-free atmosphere to reduce the numberof hydroxyl groups in the material of the formation 30. The nextfollowing step is the sintering of the tubular formation 30, which isconducted at a temperature of about between 1200° and 1400° C. In thissintering operation, the particles of the formation 30 will fuse withone another, thus eliminating most if not all of the pores of theformation 30. Yet, the formation still retains its tubular shape. Then,finally, the formation 30 is heated to a temperature of about 2000° C.,resulting in an inward collapse of the formation 30 and in eliminationof the remainder of the pores, if any, so that a solid rod-shapedoptical preform is obtained.

The optical preform can then either be permitted to cool and be storedfor future use or immediately used for drawing the optical fibertherefrom. The latter approach has the advantages not only ofeliminating the need for additional handling steps between the formationof the optical preform and the drawing of the fiber therefrom, but alsoof utilizing at least a part of the heat content imparted to the preformduring the previous pre-heating,, sintering and collapsing steps in thedrawing operation. This results in a very economical operation whichreflects itself in the manufacturing cost of the fiber.

The succession of steps involved in the production of an optical cablefrom the starting materials is diagrammatically illustrated in FIG. 3.

Another method of slurry deposition will be to traverse the rotatingbait rod 20, keeping the spray gun position fixed. Other methods can beinstituted to make optical fiber preform by the novel process describedherein. The essential components of this process are to make a slurryout of silica based oxides and an organic vehicle system appropriate forthe deposition of the particulate oxides in a dimensionally stable format room temperature. The deposited materials will dry up instantaneouslyand sufficiently to assume the appearance of a solid layer which willhave strong adherence to each other but not to the bait surface. Theorganic binder in this system will provide high strength to hold thesolid particles together after the deposition and thus maintain precisedimensional control of the slurry preform. In essence, Layer SlurryDeposition (LSD) process has the following advantages.

Fibers of any required design can be processed due to precise control ofrefractive index profile.

Large preforms and hence long length fibers can be drawn.

High deposition rate and efficiency, low loss of raw materials and lessexpensive unit operations will make the fibers more cost-effective.

Because the system is non-aqueous and entirely organic, the chances ofOH contamination during the deposition process is almost none.

The depositing operation can be conducted at room temperature.

While we have described above the principles of our invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of our invention as set forth in the objects thereof and inthe accompanying claims.

We claim:
 1. A process for manufacturing an optical preform, comprisingthe steps ofproviding a bait; forming at least one slurry containingparticles of an optical material with a predetermined index ofrefraction suspended in a liquid vehicle, including coating theparticles with a binder; depositing at least one cohesive layer of theparticles on the bait, including covering at least a portion of the baitwith the slurry and evaporating the liquid vehicle while keeping thebinder for the latter to hold the particles together in the cohesivelayer subsequent to the evaporation of the liquid vehicle; dissociatingthe cohesive layer from the bait; removing the binder from the cohesivelayer; and sintering the cohesive layer into the preform.
 2. The processas defined in claim 1, whereinsaid depositing step includes directing atleast one stream of the slurry against the bait.
 3. The process asdefined in claim 2, whereinsaid depositing step further includesconducting relative movement between the stream and the bait.
 4. Theprocess as defined in claim 3 for use with an elongated bait having acircumferential surface centered on an axis, whereinsaid conducting stepincludes performing relative movement between the bait and the stream ofthe slurry both in the axial and in the circumferential direction of thebait.
 5. The process as defined in claim 4, whereinsaid performing stepincludes rotating the bait about said axis and moving the stream axiallyof the bait.
 6. The process as defined in claim 1, and furthercomprising the step of repeatingsaid depositing step to deposit anadditional cohesive layer of the particles on top of the cohesive layerdeposited during the previous depositing step.
 7. The process as definedin claim 6, whereinsaid forming step includes forming a plurality ofslurries containing particles of optical materials with differentindexes of refraction including the predetermined index of refraction;and wherein said repeating step includes using a slurry selected fromthe plurality of slurries which contains particles of a different indexof refraction from those deposited during the previous depositing step.8. The process as defined in claim 1, whereinsaid removing step includesheating at least the cohesive layer to a temperature sufficient to expelthe binder from the cohesive layer.
 9. The process as defined in claim1; and further comprising the step of heating at least the cohesivelayer in a hydrogen-free atmosphere prior to said sintering step toreduce the hydroxyl contents of the cohesive layer.
 10. An apparatus formanufacturing an optical preform, comprisingmeans for mounting a bait;means for supplying at least one slurry containing particles of anoptical material with a predetermined index of refraction suspended in aliquid vehicle and coated with a binder; means for depositing at leastone cohesive layer of the particles on the bait, including means forcovering at least a portion of the bait with the slurry from which theliquid vehicle is removed by evaporation while the binder remainstherein to hold the particles together in the cohesive layer subsequentto the evaporation of the liquid vehicle; means for removing the binderfrom the cohesive layer after its dissociation from the bait; and meansfor sintering the cohesive layer subsequent to the removal of the binderinto the preform.
 11. The apparatus as defined in claim 10, whereinsaiddepositing means further includes means for directing at least onestream of the slurry against the bait.
 12. The apparatus as defined inclaim 11, whereinsaid depositing means further includes means forconducting relative movement between said directing means and the bait.13. The apparatus as defined in claim 12 for use with an elongated baithaving a circumferential surface centered on an axis, whereinsaidconducting means includes means for performing relative movement betweenthe bait and the directing means both in the axial and in thecircumferential direction of the bait.
 14. The apparatus as defined inclaim 13, whereinsaid mounting means mounts the bait for rotation aboutsaid axis; and wherein said performing means includes means for rotatingthe bait, and means for moving said directing means axially of the bait.